A magnetic sensor is a sensor that uses a magnetoresistance element having a property in which the resistance varies according to magnetic field strength. The magnetization of a magnetic pattern included in a sheet-like medium such as paper currency is typically small. For this reason, particularly in a line-type magnetic sensor that detects multiple channels simultaneously, an anisotropic magnetoresistance element is often used in order to detect a magnetic pattern with high sensitivity. An anisotropic magnetoresistance element generally is more sensitive than a semiconductor magnetoresistance element. In a magnetic sensor, it is necessary to provide anisotropic magnetoresistance elements in areas of magnetic field strength where the sensitivity rises without any reaching magnetic saturation, and then convey a sheet-like medium such as paper currency through the strong magnetic field range.
However, in a magnetic sensor using an anisotropic magnetoresistance element, since an anisotropic magnetoresistance element saturates at a magnetic flux density of approximately 10 mT, it is difficult to dispose multiple anisotropic magnetoresistance elements in areas of magnetic field strength where the sensitivity rises without saturating.
In order to solve such a problem, Patent Literature 1 discloses a magnetic sensor in which a permanent magnet is disposed with the position adjusted so that the bias magnetic field strength in the magneto-sensing direction of ferromagnetic thin film magnetoresistance elements (anisotropic magnetoresistance elements) simultaneously imparted by a detecting magnetic field from the permanent magnet reaches a magnetic flux that is not greater than the saturation magnetic field.
The magnetic sensor in Patent Literature 1 disposes a magnetoresistance element between a magnet and the object of detection. Besides the above, there are magnetic sensors configured so that the object of detection is moved between a magnet and a magnetoresistance element (see Patent Literature 2, for example), or, configured so that a magnetoresistance element is disposed between two magnets, and the object of detection is moved between the magnetoresistance element and one of the magnets (see Patent Literature 3, for example). With the configuration that moves the object of detection between a magnet and a magnetoresistance element, sometimes a magnet and a magnetized body are disposed opposite each other, and a magnetoresistance element is disposed on the side of the magnetized body.
For example, in the magnetic sensor in Patent Literature 4, a permanent magnet and a magnetized yoke are disposed opposite each other with a travel space therebetween, with the permanent magnet disposed below and the magnetized yoke disposed above. The opposing magnetic pole face of the permanent magnet forms the same magnetic polarity. A magnetoresistance element constituting a voltage-dividing circuit is installed on the side of the opposing face of the magnetized yoke. A groove is formed in the opposing face of the magnetized yoke. Lines of magnetic force coming from the permanent magnet converge and enter the magnetic pole of the magnetized yoke, and by disposing the magnetoresistance element in this magnetic flux convergence area, the resolution at which an object may be detected is raised.
Also, in the magnetic sensor of Patent Literature 5, the magnetized yoke is disposed above the magnet. A magnetic leg portion of the magnetized yoke has shape magnetic anisotropy, which suppresses the spread of the magnetic flux of the magnet.